H1N1 deaths confirmed in the global pandemic three years ago stood at 18,500 but the true number could be between 151,700 and 575,400.

About 80 per cent of those who died were people aged under 65, and 59 per cent of deaths occurred in south-east Asia and Africa, according to research published in the Lancet.

Report author Dr Fatimah Dawood, of the US Centers for Disease Control and Prevention, said: ‘The study underscores the significant human toll of an influenza pandemic.’

The latest estimates were based on data from countries that held information on people who developed flu symptoms, as well as the number of deaths among flu cases during the pandemic.

But a lack of information was particularly pronounced for low-and middle-income countries.

Another of the report’s authors, Dr Cecile Viboud of the US-based National Institutes of Health, said: ‘These results are likely to be refined as more studies from low-income and middle-income regions become available, particularly from China and India.’